"Soft" quartz glass has lower viscosity than pure quartz glass. For many applications, the very low coefficient of thermal expansion of quartz glass is desirable, without, however, requiring the characteristics of pure quartz glass, namely that it can accept high thermal loading. High thermal loading is usually associated with high viscosity. The advantage of soft quartz glass is that its shaping and handling during manufacture is simpler and requires less energy than the highly viscous quartz glasses. Usually, the characteristics of soft quartz glass are obtained by mixing ultra-pure quartz glass with small quantities of alkali oxides and alkaline earth oxides. Additional additives of aluminum oxide, Al.sub.2 O.sub.3, titanium oxide, TiO.sub.2, and/or zirconium oxide, ZrO.sub.2, may be used, see the referenced U.S. Pat. No. 4,336,048, van der Steen. By carefully controlling the mixture of these components and matching the components in the mixture, a small addition of, for example, less than 0.5 mol-% can be used to obtain a viscosity which is similar to that of the known, substantially higher doped Vycor.TM. glass. Vycor, besides alkali oxides further contains about 3% of B.sub.2 O.sub.3, the remainder being silicon dioxide. For high-purity or ultra-purity quartz glass, the starting material should be pure to, or not substantially less pure than, about 99.99 mol-% SiO.sub.2.
The known quartz glass has a disadvantage, however, in that the alkali oxides tend to vaporize at the high temperature prevalent in a glass furnace. Thus, as it is handled and worked on, it becomes more viscous, and tougher. Alkali oxide additives also lead to problems in some uses, where particularly high purity is important or the electrical insulation characteristic is important. For example, when making envelopes or vessels for discharge lamps, the plasma in the discharge lamp may be subject to undesired contamination. In highly loaded incandescent lamps, spurious discharges between conductors may occur due to alkali-ion charge carriers.